Mining for ore isn’t what it used to be. A hundred years ago when a
mining operation happened upon a copper vein or a gold deposit, the outfit would
often just rip out the local flora, excavate the land, and dump the resulting
waste and rubble in the nearest pristine stream or flowering meadow. But more
often than not such mining practices contaminated the surrounding soil with
heavy metals or raised the acidity in nearby lakes to levels lethal for fish.
Consequently, mining operations in most developed countries must now follow
strict guidelines and stay away from environmentally protected areas, dispose of
their waste safely, and restore the land to a close approximation of its
original state when finished.

Though these regulations have done wonders for the environment in places such
as in the American West, companies must now spend a fortune even to locate areas
suitable for digging. To aid mining operations in their search for suitable
sites and to simply keep a long-term record of the world’s resources, the
U.S. Geological Survey (USGS) two years ago initiated the Global Mineral
Resource Assessment Project. The goal of the project is to construct a series of
reports outlining where the world’s known metal ore deposits such as gold,
copper, and iron occur and where new deposits are likely to be found.

As part of this ambitious effort, geologist Larry Rowan and a team of
researchers at the USGS are now developing ways to locate potentially
mineralized areas from space. Using the new Advanced Spaceborne Thermal
Emissions and Reflection Radiometer (ASTER) instrument aboard the Terra
satellite, they are attempting to find areas likely to contain copper in Iran
and western Pakistan. If their tests are successful, then ASTER and future
satellites could aid geologists in pinpointing any number of metal ores from
aluminum to tin.

Tailing piles rise beside the abandoned buildings
of the Kennecott Copper Mine in Alaska. This mine site was discovered when prospectors spied unusual
green rocks. New satellites are allowing modern-day prospectors to map mineral deposits from afar.
(Image courtesy Historic American Engineering Record)

“For now, ASTER is really the only practical solution. It is the only
instrument up there with the combination of spectral capability and geographic
coverage to locate potentially mineralized areas on a global or regional
scale,” says Rowan. He explains that mapping the world’s minerals
could not be accomplished on the ground or by airplane alone. The costs would be
enormous. The only feasible way to carry out such a large survey is through the
use of remote sensing satellites. The problem with previous satellite
instruments, however, was that while they could view large sections of the
Earth, they didn’t have the ability to detect the geological information
needed to reveal metal ore.

Some of the oldest known and best-mapped
copper deposits in the world are located along a swath of land extending from central Iran to western Pakistan.
Copper has been mined in the region for over 6,500
years. Currently, scientists from the U.S. Geological Survey are correlating Advanced
Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data with these
deposits. When these methods are refined, the scientists will be able to map
potential copper deposits worldwide. (Map adapted from Larry Rowan, USGS)

The ASTER instrument, launched aboard NASA’s Terra satellite in
December 1999, was designed in part to remedy this problem. The instrument moves
in a nearly circular orbit approximately from pole to pole around the Earth and
measures a wide range of visible and infrared solar radiation from the surface
of our planet. ASTER has a spatial resolution of up to 15 meters and can gather
stereoscopic (3-D) images of the Earth, making it ideal for mapping mountains
and rock formations.

Rowan and the USGS team decided to test ASTER’s abilities in mapping
copper ore. Used in everything from electrical systems to compact discs to air
conditioners, copper is one of the more expensive and useful non-precious
metals. As a proving grounds, the USGS team turned their attention to a string
of known copper deposits located across a 500-mile-long arc that extends from
the northwest corner of Iran through the Zagros Mountains and into western
Pakistan. Since this area is cloud free most of the year and many of the
deposits have been mined since the beginning of the Bronze Age, the region is
ideal for trying out ASTER.

“The types of deposits that lend themselves to remote sensing in Iran
are intrusive deposits,” says Rowan. He explains that intrusive ore
deposits form deep underground. Magma from the Earth’s mantle percolates
into the Earth’s crust in areas where two tectonic plates collide or above
“hotspots” in the mantle. As this magma solidifies to create rocks
such as granite, the intense heat and pressure sometimes acts in concert with
underground water to forge copper deposits at the boundary between the cooling
magma and the existing crust. In Iran, the intrusive deposits first took shape
between 10 and 70 million years ago at the boundary of the Arabian Plate and the
Eurasian Plate. The copper deposits formed thousands of feet under the
Earth’s surface. Continued uplift and years and years of water, ice, and
wind erosion have exposed the copper ore to the Earth’s surface for
mining.

In looking for such intrusive deposits, geologists must focus in on those
large features such as faults and folds and mountains that indicate possible
ore-bearing intrusive rocks. In ASTER images, such large-scale features are
usually readily apparent. Narrowing the search to specific locations among these
intrusive rocks, however, is considerably more involved. Like most metals,
copper doesn’t exist in abundance on the Earth’s surface. In fact,
copper only makes up 0.00007 percent of the Earth’s crust. So even in
areas where deposits are common, copper is not easy to spot.

“What we do is look for other minerals that generally indicate that
copper is in the region,” says Rowan. Fortunately, the same geothermal
processes that create copper deposits usually give rise to other minerals, known
as alteration minerals, that tend to be more abundant and more noticeable.
Minerals such as muscovite, alunite, kaolnite, and iron sulfide are typically
found in abundance around copper deposits. Some of these, such as iron sulfide,
react with surface water and oxygen to form highly visible markers (Rowan et al.
2002). “If you’ve ever been to an old mining site in Colorado, the
presence of the resulting sulfates can be seen in the yellowish to brown red
cast they leave in the water,” says Rowan.

Though uncovering alteration minerals is much easier than finding an isolated
copper vein, geologists still must go around collecting, testing for, and
mapping the alteration minerals in large rock formations. ASTER, however, should
be able to significantly reduce the amount of work and cost involved in this
process. All told, ASTER can detect 14 different wavelengths (colors) of light
reflected and emitted from the surface of the Earth. Generally, everything on
the Earth’s surface reflects certain colors of light and absorbs others.
Green grass, for instance, absorbs all colors in the visible spectrum except
green. Alteration minerals, though usually much more subtle in color, absorb
and reflect specific bands as well. Rowan explains that nine of the satellite
instrument’s sensors that detect light in the visible and near-infrared
(light past red on the color spectrum) range are particularly sensitive to the
wavelengths associated with alteration minerals.

The ASTER image on the
left is a composite of visible and near-infrared wavelengths, while on the right
three shortwave infrared bands are used. Colors in the shortwave infrared composite
correspond to different rock types. Pink indicates sericite (similar to muscovite), a
component of altered granite that is associated with copper ore. (Images courtesy Mike Abrams, ASTER Science Team, JPL)

Intrusive rocks—such as this muscovite granite—form
from magma that solidifies beneath the Earth’s surface. Over millions of years scalding hot water leaches
copper and other metals out of the rocks and deposits them in veins of ore. Later, erosion exposes the rocks on
the surface. (Photograph from Geotectonics Field Trip, 2000 copyright Dave Kimbrough, San Diego State University)

The rocks in this image are
stained by limonite, formed by the interaction of oxygen in the air with an iron sulfide named pyrite.
Iron sulfide minerals and their derivatives are some of the markers that may indicate copper deposits. (Photograph courtesy Larry Rowan, USGS)

The iron sulfide
minerals associated with copper deposits and other ores stain water bright orange.
This Missouri stream is colored by runoff from a nearby mine. (Photograph from “The Status and Trends of the Nation’s Biological Resources” courtesy USGS)

“One example of the minerals we classify that indicates the possible
presence of copper is muscovite,” says Rowan. Muscovite is a clear mineral
with up to a quarter inch thickness that peels apart in thin layers. Back in the
days before tempered glass, the mineral was used for oven windows (Isinglass).
Though most visible wavelengths pass right through the mineral, it absorbs at a
wavelength in the near-infrared part of the spectrum. This is the same
wavelength that band 6 on ASTER detects. To map out the levels of muscovite in
a region, scientists measure the brightness of light hitting band 6 on ASTER.
The lower the readings are (i.e., the less light that is reflected back to the
sensor) in an area, the more muscovite there is likely to be.

Alteration minerals are found in areas where the geologic history could have produced
copper deposits. The presence of these minerals alone, however, does not guarantee the presence of copper. [Photographs from
the Ko-collection (Japanese), copyright Kyushu University]

“I caution that muscovite can occur in other rock types. This is not a
formulation that will work without ambiguity. We must combine this with other
information such as the rocks from different geological maps and look for
certain ages and combinations of minerals that are favorable for copper,”
says Rowan. While muscovite is one key ingredient indicating the presence of
copper, there are a number of other minerals that the team must locate to narrow
the search. Since many of these alteration minerals can be found in differing
proportions around other metals, such as gold, the team has to take the relative
abundance of the minerals into account as well. The researchers then often need
data on the age of the rock beds, the structure of the rock formations, and even
the thickness of the surrounding vegetation. They bring all of this information
together onto one single digitized map using Geographic Information System (GIS)
software.

Rowan points out that though they are still in the preliminary stages of
mapping potential copper deposits in Iran, so far their results seem to match up
with known locations very well. If the mapping proves to be successful, then the
USGS will employ ASTER to map not just copper, but gold, iron, and other ore
deposits in well exposed areas all over the world. Such maps could help mining
companies more cost effectively locate minerals in ways that will not disrupt
the ecosystem. Such maps could also be used by the United States to search for
potential sources of minerals such as platinum that are hard to find in North
America. And poorer countries around the world could utilize these maps to tap
into their resources and jumpstart their economies. “It should improve the
efficiency and cost of such projects all around the world,” says Rowan.